plant science research

Crab spiders that hunt in flowers prey on pollinating insects. Thus, pollinating insects tend to avoid flowers that harbor crab spiders. We established this in part one. Now we ask, what effect, if any, does this interaction have on a crab spider infested plant’s ability to reproduce? More importantly, what are the evolutionary implications of this relationship?

In a study published in Ecological Entomologyearlier this year, Gavini, et al. found that pollinating insects avoided the flowers of Peruvian lily (Alstroemeria aurea) when artificial spiders of various colors and sizes were placed in them. Bumblebees and other bees were the most frequent visitors to the flowers and were also the group “most affected by the presence of artificial spiders, decreasing the number of flowers visited and time spent in the inflorescences.” This avoidance had a notable effect on plant reproduction, namely a 25% reduction in seed set and a 15% reduction in fruit weight. The most abundant and effective pollinator, the buff-tailed bumblebee, was deterred by the spiders, leading the researchers to conclude that, “changes in pollinator behavior may translate into changes in plant fitness when ambush predators alter the behavior of the most effective pollinators.”

But missing from this discussion is the fact that crab spiders don’t only eat pollinators. Any flower visiting insect may become a crab spider’s prey, and that includes florivores. In which case, crab spiders can benefit a plant, saving it from reproduction losses by eating insects that eat flowers.

In April of this year, Nature Communications published a study by Knauer, et al. that examined the trade-off that occurs when crab spiders are preying on both pollinators and florivores. Four populations of buckler-mustard (Biscutella laevigata ssp. laevigata) were selected for this study. Bees are buckler-mustard’s main pollinator, and in concurrence with other studies, they significantly avoided flowers when crab spiders were present. Knauer, et al. also determined that bees and crab spiders are attracted to the same floral scent compound, β-ocimene. This compound not only attracts pollinators, but is also emitted when plants experience herbivory, possibly to attract predators to come and prey on whatever is eating them.

In this study, the predators called upon were crab spiders. Florivores had a notable impact on plants in this study, and the researchers found that when crab spiders were present, florivores were significantly reduced, thereby reducing their negative impact. They also noted that “crab spiders showed a significant preference for [florivore-infested] plants over control plants.”

And so it is, a plant’s floral scent compound attracts pollinators while simultaneously attracting the pollinator’s enemy, who is also called in to protect the flower from being eaten. Luckily, in this case, buckler-mustard is easily pollinated, so the loss of a few pollinators isn’t likely to have a strong negative effect on reproduction. As the authors write, “pollinators are usually abundant and the low number of ovules per flower makes a few pollen grains sufficient for a full seed set.”

crab spider on zinnia

But none of these studies are one size fits all. Predator-pollinator-plant interactions are still not well understood, and there is much to learn through future research. A meta-analysis published in the Journal of Animal Ecology in 2011 looked at the research that had been done up to that point. Included were a range of studies involving sit-and-wait predators (like crab spiders and lizards) as well as active hunters (like birds and ants) and the effects of predation on both pollinators and plant-eating insects. They concluded that where carnivores “disrupted plant-pollinator interactions, plant fitness was reduced by 17%,” but thanks to predation of herbivores, carnivores helped increase plant fitness by 51%. This suggests that carnivores, overall, have a net positive effect on plant fitness.

Many pollinating insects have an advantage over plant-eating insects because they move quickly from flower to flower and plant to plant, unlike many herbivores which move more slowly. This protects pollinators from predation and helps explain why plant-pollinator interactions are not disrupted as easily by carnivores. Additionally, as the authors note, “plants may be buffered against loss of pollination by attracting different types of pollinators, some of which are inaccessible to carnivores.”

But again, there is still so much to discover about these complex interactions. One way to gain a better understanding is to investigate the effects of predators on both pollinators and herbivores in the same study, since many of the papers included in the meta-analysis focused on only one or the other. As far as crab spiders go, Knauer, et al. highlight their importance in such studies. There are so many different species of crab spiders, and they are commonly found on flowers around the globe, so “their impact on plant evolution may be widespread among angiosperms.”

In other words, while we still have a lot to learn, the impact these tiny but skillful hunters have should not be underestimated.

When a bee approaches a flower, it is essentially approaching the watering hole. It comes in search of food in the form of pollen and nectar. As is this case with other animals who come to feed at the watering hole, a flower-visiting bee makes itself vulnerable to a variety of predators. Carnivores, like the crab spider, lie in wait to attack.

The flowers of many plants rely on visits from bees and other organisms to assist in transferring pollen from stamens to stigmas, which initiates reproduction; and bees and other flower visitors need floral resources to survive. Crab spiders exploit this otherwise friendly relationship and, in doing so, can leave lasting impacts on both the bees and the flowers they visit.

Species in the family Thomisidae are commonly referred to as crab spiders, a name that comes from their resemblance to crabs. Crab spiders don’t build webs to catch prey; instead they either actively hunt for prey or sit and wait for potential prey to happen by, earning them the name ambush predators. Of the hundreds of species in this family, not all of them hunt for prey in flowers; those that do – species in the genera Misumena and Thomisus, for example – are often called flower crab spiders.

Most crab spiders are tiny – mere millimeters in size – and they have a number of strategies (depending on the species) to obscure their presence from potential prey. They can camouflage themselves by choosing to hunt in a flower that is the same color as they are or, in the case of some species, they can change their color to match the flower they are on. Some species of crab spiders reflect UV light, which bees can see. In doing so, they make themselves look like part of the flower.

Using an Australian species of crab spider, researchers foundthat honey bees preferred marguerite daisies (Chrysanthemum frutescens) on which UV-reflecting crab spiders were present, even when the scent of the flowers was masked. The spiders’ presence was seen as nectar guides, which “bees have a pre-existing bias towards.” Members of this same research team also determined that both crab spiders and honey bees choose fragrant flowers over non-fragrant flowers, and that, ultimately, “honey bees suffer apparently from responding to the same floral characteristics as crab spiders do.”

Needless to say, crab spiders are crafty. So the question is, when killing machines like crab spiders are picking off a plant’s pollinators, does this affect its ability to reproduce? First let’s consider how pollinators react to finding crab spiders hiding in the flowers they hope to visit.

A study published in Oikos in 2003 observed patches of common milkweed (Asclepias syriaca) – one set was free of crab spiders, the other set was not – and tracked the visitations of four species of bees – the common honey bee and three species of bumble bees. They compared visitation rates between both sets of milkweed patches and found that the smallest of the three bumble bee species decreased its frequency of visitation to the crab spider infested milkweeds. Honey bees also appeared to visit the infested milkweeds less, but the results were not statistically significant. The two larger species of bumble bees continued to forage at the same rate despite the presence of crab spiders.

During the study, crab spiders were seen attacking bees numerous times. Six attacks resulted in successful kills, and of the bees that escaped, 80% left the flower and either moved to a different flower on the same plant, moved to a different plant, or left the patch altogether. These results indicate a potential for the presence of crab spiders to effect plant-pollinator interactions, whether its directly (predation) or indirectly (bees avoiding flowers with crab spiders).

Another study published in Behavioral Ecologyin 2006 looked at two species of bees – the honey bee and a species of long-horned bee – and their reactions to the presence of crab spiders on the flowers of three different plant species – lavender (Lavandula stoechas), crimson spot rockrose (Cistus ladanifer), and sage-leaf rockrose (Cistus salvifolius). Honey bees were about half as likely to select inflorescences of lavender when crab spiders were present, and they avoided the crab spider infested flowers of crimson spot rockrose with a similar frequency. On the other hand, the long-horned bee visited the flowers of crimson spot rockrose to the same degree whether or not a crab spider was present.

The researchers then exposed honey bees to the flowers of sage-leaf rockrose that were at the time spider-free but showed signs that crab spiders had recently visited. Some of the flowers featured the scent of crab spiders, others had spider silk attached to them, and others had the corpses of dead bees on them. They found that even when crab spiders were no longer present, the bees could still detect them. Honey bees were particularly deterred by the presence of corpses. The long-horned bees were also exposed to the flowers with corpses on them but didn’t show a significant avoidance of them.

An interesting side note about the presence of silk on flowers. As stated earlier, crab spiders do not spin webs; however, they do spin silk for other reasons, including to tether themselves to flowers while hunting. The authors recount, “on several occasions when an attempted attack was observed during this study, it was only the presence of a silk tether that prevented spiders being carried away from flowers by their much larger prey.”

So, again, if bees are avoiding flowers due to the presence of predators like crab spiders, what effect, if any, is this having on the plants? We will address this question in part two.

Earlier this month, I went to Garden Grove, California to attend the 71st annual meeting of the Western Society of Weed Science. My trip was funded by an Education and Enrichment Award presented by the Pahove Chapterof the Idaho Native Plant Society. It was a great opportunity for a weeds-obsessed plant geek like myself to hang out with a bunch of weed scientists and learn about their latest research. What follows are a few highlights and takeaways from the meeting.

General Session

Apart from opening remarks and news/business-y stuff, the general session featured two invited speakers: soil ecologist Lydia Jennings and historian David Marley. Lydia’s talk was titled “Land Acknowledgement and Indigenous Knowledge in Science.” She started by sharing a website called Native Land, which features an image of the Earth overlayed with known “borders” of indigenous territories. By entering your address, you can see a list of the tribes that historically used the land you now inhabit. It is important for us to consider the history of the land we currently live and work on. Lydia then compared aspects of western science and indigenous science, pointing out ways they differ as well as ways they can be used in tandem. By collaborating with tribal nations, weed scientists can benefit from traditional ecological knowledge. Such knowledge, which has historically gone largely unrecognized in the scientific community, should receive more attention and acknowledgement.

David Marley was the comic relief. Well-versed in the history of Disneyland, he humorously presented a series of stories involving its creation. Little of what he had to say related to weed science, which he openly admitted along the way; however, one weeds related story stood out. Due to a lack of funds, the early years of Tomorrowland featured few landscape plants. To make up for that, Walt Disney had signs with fake Latin names created for some of the weeds.

Weeds of Range and Natural Areas

I spent the last half of the first day in the “Weeds of Range and Natural Areas” session where I learned about herbicide ballistic technology (i.e. killing plants from a helicopter with a paintball gun loaded with herbicide). This is one of the ways that Miconia calvescens invasions in Hawaii are being addressed. I also learned about research involving plant debris left over after logging. When heavy amounts of debris are left in place, scotch broom (Cytisus scoparius) infestations are thwarted. There was also a talk about controlling escaped garden loosestrife (Lysimachia punctata) populations in the Seattle area, as well as a few talks about efforts to control annual grasses like cheatgrass (Bromus tectorum) in sagebrush steppes. Clearly there are lots of weed issues in natural areas, as that only covers about half the talks.

Basic Biology and Ecology

On the morning of the second day, the “Basic Biology and Ecology” session held a discussion about weeds and climate change. As climate changes, weeds will adapt and find new locations to invade. Perhaps some weeds won’t be as problematic in certain areas, but other species are sure to take their place. Understanding the changes that are afoot and the ways that weeds will respond to them is paramount to successful weed management. This means documenting the traits of every weed species, including variations between and among populations of each species, so that predictions can be made about their behavior. It also means anticipating new weed species and determining ways in which weeds might exploit new conditions.

No doubt there is much to learn in order to adequately manage weeds in a changing climate. An idea brought up during the discussion that I was particularly intrigued by was using citizen scientists to help gather data about weeds. Similar to other organizations that collect phenological data from the public on a variety of species, a website could be set up for citizen scientists to report information about weeds in their area, perhaps something like this project in New Zealand. Of course, there are already a series of apps available in North America for citizen scientists to report invasive species sightings, so it seems this is already happening to some degree.

Teaching and Technology Transfer

A highlight of the afternoon’s “Teaching and Technology Transfer” session was learning about the Wyoming Restoration Challenge hosted by University of Wyoming Extension. This was a three year long contest in which thirteen teams were given a quarter-acre plot dominated by cheatgrass with the challenge to restore the plant community to a more productive and diverse state. Each team developed and carried out their own strategy and in the end were judged on a series of criteria including cheatgrass and other weed control, plant diversity, forage production, education and outreach, and scalability. Preliminary results can be seen here; read more about the challenge here and here.

And so much more…

Because multiple sessions were held simultaneously, I was unable to attend every talk. I also had to leave early on the third day, so I missed those talks as well. However, I did get a chance to sit in on a discussion about an increasingly troubling topic, herbicide-resistant weeds, which included a summary of regional listening sessionsthat have been taking place in order to bring more attention to the subject and establish a dialog with those most affected by it.

The lettuce we eat is a close relative to the lettuce we weed out of our gardens. Last week we discussed the potential that wild relatives may have for improving cultivated lettuce. But if wild lettuce can be crossed with cultivated lettuce to create new cultivars, can cultivated lettuce cross with wild lettuce to make it more weedy?

Because so many of our crops are closely related to some of the weeds found along with them or the plants growing in nearby natural areas, the creation of crop-wild hybrids has long been a concern. This concern is heightened in the age of transgenic crops (also known as GMOs), for fear that hybrids between weeds and such crops could create super weeds – fast spreading or highly adapted weeds resistant to traditional control methods such as certain herbicides. To reduce this risk, extensive research is necessary before such crops are released for commercial use.

flowers of prickly lettuce (Lactuca serriola)

There are no commercially available, genetically modified varieties of cultivated lettuce, so this is not a concern when it comes to crop-wild hybrids; however, due to how prevalent weedy species like prickly lettuce (Lactuca serriola) are, hybridization with cultivated lettuce is still a concern. So, it is important to understand what the consequences might be when hybridization occurs.

In a paper published in Journal of Applied Ecology in 2005, Hooftman et al. examined a group of second-generation hybrids (L. sativa x L. serriola), and found that the hybrids behaved and appeared very similarly to non-hybrid prickly lettuce. They also found that the seeds produced by the hybrids had a significantly higher germination rate than non-hybrid plants. This is an example of hybrid vigor. Thus, “if hybridization does occur, this could lead to better performing and thus potentially more invasive (hybrid) genotypes.” However, the authors cautioned that “better performing genotypes do not automatically result in higher invasiveness,” and that much depends on the conditions they are found in, the level of human disturbance, etc.

Another thing to consider is that hybrids are not stable. In an article published in Nature Reviews Geneticsin 2003, Stewart et al. adress the “misunderstanding that can arise through the confusion of hybridization and … introgression.” It is wrong to assume that hybrids between crops and wild relatives will automatically lead to super weeds. For this to occur, repeated crosses with parental lines (also known as backcrossing) must occur, and “backcross generations to the wild relative must progress to the point at which the transgene [or other gene(s) in question] is incorporated into the genome of the wild relative.” That is what is meant by “introgression.” This may happen quickly or over many generations or it may never happen at all. Each case is different.

prickly leaf of prickly lettuce (Lactuca serriola)

In a paper published in Journal of Applied Ecology in 2007, Hooftman et al. observe the breakdown of crop-wild lettuce hybrids. They note that “fitness surplus through [hybrid vigor] will often be reduced over few generations,” which is what was seen in the hybrids they observed. One possible reason why this occurs is that lettuce is predominantly a self-crossing species; outcrossing is rare, occurring 1 – 5% of the time thanks to pollinating insects. But that doesn’t mean that a stable, aggressive genotype could never develop. Again, much depends on environmental conditions, as well as rates of outcrossing and other factors relating to population dynamics.

A significant expansion of prickly lettuce across parts of Europe led some to hypothesize that crop-wild hybrids were partly to blame. In a paper published in Molecular Ecologyin 2012 Uwimana et al. ran population genetic analyses on extensive data sets to determine the role that hybridization had in the expansion. They concluded that, at a level of only 7% in wild habitats, crop-wild hybrids were not having a significant impact. They observed greater fitness in the hybrids, as has been observed in other studies (including the one above), but they acknowledged the instability of hybrids, especially in self-pollinating annuals like lettuce.

seed head of prickly lettuce (Lactuca serriola)

It is more likely that the expansion of prickly lettuce in Europe is due to “the expansion of favorable habitat as a result of climate warming and anthropogenic habitat disturbance and to seed dispersal because of transportation of goods.” Uwimana et al. did warn, however, that “the occurrence of 7% crop-wild hybrids among natural L. serriola populations is relatively high [for a predominantly self-pollinating species] and reveals a potential [for] transgene movement from crop to wild relatives [in] self-pollinating crops.”

Lettuce, domesticated about six thousand years ago in a region referred to as the Fertile Crescent, bears little resemblance to its wild ancestors. Hundreds of years of cultivation and artificial selection eliminated spines from the leaves, reduced the latex content and bitter flavor, shortened stem internodes for a more compact, leafy plant, and increased seed size, among several other things. The resulting plant even has a different name, Lactuca sativa (in Latin, sativa means cultivated). However, cultivated lettuce remains closely related to its progenitors, with whom it can cross to produce wild-domestic hybrids. For this reason, there is great interest in the wild relatives of lettuce and the beneficial traits they offer.

Crop wild relatives are a hot topic these days. That’s because feeding a growing population in an increasingly globalized world with the threat of climate change looming requires creative strategies. Utilizing wild relatives of crops in breeding programs is a potential way to improve yields and address issues like pests and diseases, drought, and climate change. While this isn’t necessarily a new strategy, it is increasingly important as the loss of biodiversity around the globe threatens many crop wild relatives. Securing them now is imperative.

There are about 100 species in the genus Lactuca. Most of them are found in Asia and Africa, with the greatest diversity distributed across Southwest Asia and the Mediterranean Basin. The genus consists of annual, biennial, and perennial species, a few of which are shrubs or vines. Prickly lettuce (L. serriola), willowleaf lettuce (L. saligna), and bitter lettuce (L. virosa) are weedy species with a wide distribution outside of their native range. Prickly lettuce is particularly common in North America, occurring in the diverse habitats of urban areas, natural areas, and agricultural fields. It is also the species considered to be the main ancestor of today’s cultivated lettuce.

In a paper published in European Journal of Plant Pathology in 2014. Lebeda et al. discuss using wild relatives in lettuce breeding and list some of the known cultivars derived from crosses with wild species. They write that in the last thirty years, “significant progress has been made in germplasm enhancement and the introduction of novel traits in cultivated lettuce.” Traditionally, Lactuca serriola has been the primary source for novel traits, but breeders are increasingly looking to other species of wild lettuce.

Resistance to disease is one of the main aims of lettuce breeders. Resistance genes can be found among populations of cultivated lettuce, but as “extensive screening” for such genes leads to “diminishing returns in terms of new resistance,” breeders look to wild lettuce species as “sources of new beneficial alleles.” The problem is that there are large gaps in our knowledge when it comes to wild lettuce species and their interactions with pests and pathogens. Finding the genes we are looking for will require “screening large collections of well defined wild Lactuca germplasm.” But first we must develop such collections.

In a separate paper (published in Euphytica in 2009), Lebeda et al. discuss just how large the gaps in our understanding of the genus Lactuca are. Beginning with our present collections they found “serious taxonomic discrepancies” as well as significant redundancy and unnecessary duplicates in and among gene banks. They also pointed out that “over 90% of wild collections are represented by only three species” [the three weedy species named above], and they urged gene banks to “rapidly [acquire] lettuce progenitors and wild relatives from the probable center of origin of lettuce and from those areas with the highest genetic diversity of Lactuca species” as their potential for improving cultivated lettuce is too important to neglect.

Lactuca is a highly variable genus; species can differ substantially in their growth and phenology from individual to individual. Lebeda et al. write, “developmental stages of plants, as influenced through selective processes under the eco-geographic conditions where they evolved, can persist when plants are cultivated under common environmental conditions and may be fixed genetically.” For this reason it is important to collect numerous individuals of each species from across their entire range in order to obtain the broadest possible suite of traits to select from.

One such trait is root development and the related ability to access water and nutrients and tolerate drought. Through selection, cultivated lettuce has become a very shallow-rooted plant, reliant on regular irrigation and fertilizer applications. In an issue of Theoretical and Applied Genetics published in 2000, Johnson et al. demonstrate the potential that Lactuca serriola, with its deep taproot and ability to tolerate drought, has for developing lettuce cultivars that are more drought tolerant and more efficient at using soil nutrients.

Clearly we have long way to go in developing improved lettuce cultivars using wild relatives, but the potential is there. As Lebeda et al. write in the European Journal of Plant Pathology, “Lettuce is one of the main horticultural crops where a strategy of wild related germplasm exploitation and utilization in breeding programs is most commonly used with very high practical impact.”

Coming Up in Part Two: Can cultivated lettuce cross with wild lettuce to create super weeds?

This October 24-26th I was in Anchorage, Alaska for the 18th annual Alaska Invasive Species Workshop. The workshop is organized by the Committee for Noxious and Invasive Pests Management and University of Alaska Fairbanks Cooperative Extension. It is a chance for people involved in invasive species management in Alaska – or just interested in the topic – to learn about the latest science, policies, and management efforts within the state and beyond. I am not an Alaska resident – nor had I ever been there until this trip – but my sister lives there, and I was planning a trip to visit her and her family, so why not stop in to see what’s happening with invasive species while I’m at it?

What follows are a few highlights from each of the three days.

Day One

The theme of the workshop was “The Legacy of Biological Invasions.” Ecosystems are shaped by biotic and abiotic events that occurred in the past, both recent and distant. This is their legacy. Events that take place in the present can alter ecosystem legacies. Invasive species, as one speaker said in the introduction, can “break the legacy locks of an ecosystem,” changing population dynamics of native species and altering ecosystem functions for the foreseeable future. Alaska is one of the few places on earth that is relatively pristine, with comparably little human disturbance and few introduced species. Since they are at an early stage in the invasion curve for most things, Alaska is in a unique position to eradicate or contain many invasive species and prevent future introductions. Coming together to address invasive species issues and protect ecosystem legacies will be part of the human legacy in Alaska.

Later, Reaser gave a presentation about the National Invasive Species Council, including its formation and some of the work that it is currently doing. She emphasized that invasive species are a “people issue” – in that the actions and decisions we make both create the problem and address the problem – and by working together “we can do this.”

Day Two

Most of the morning was spent discussing Elodea, Alaska’s first invasive, submerged, freshwater, aquatic plant. While it has likely been in the state for a while, it was only recognized as a problem within the last decade. It is a popular aquarium plant that has been carelessly dumped into lakes and streams. It grows quickly and tolerates very cold temperatures, photosynthesizing under ice and snow. It propagates vegetatively and is spread to new sites by attaching itself to boats and float planes. Its dense growth can crowd out native vegetation and threaten fish habitat, as well as make navigating by boat difficult and landing float planes dangerous. Detailed reports were given about how organizations across the state have been monitoring and managing Elodea populations, including updates on how treatments have worked so far and what is being planned for the future. A bioeconomic risk analysis conducted by Tobias Schwörer was a featured topic of discussion.

After lunch I took a short break from the conference to walk around downtown Anchorage, so I missed a series of talks about environmental DNA. I returned in time to hear an interesting talk about bird vetch (Vicia cracca). Introduced to Alaska as a forage crop, bird vetch has become a problematic weed on farms, orchards, and gardens as well as in natural areas. It is a perennial vine that grows quickly, produces copious seeds, and spreads rhizomatously. Researchers at University of Alaska Fairbanks found that compared to five native legume species, bird vetch produced twice the amount of biomass in the presence of both native and non-native soil microbes, suggesting that bird vetch is superior when it comes to nitrogen fixation. Further investigation found that, using only native nitrogen-fixing bacteria, bird vetch produced significantly more root nodules than a native legume species, indicating that it is highly effective at forming relationships with native soil microbes. Additional studies found that the ability of bird vetch to climb up other plants, thereby gaining access to more sunlight and smothering host plants, contributed to its success as an invasive plant.

Seed pods of bird vetch (Vicia cracca) in Fairbanks, Alaska

Day Three

The final day of the workshop was a veritable cornucopia of topics, including risk assessments for invasive species, profiles of new invasive species, updates on invasive species control projects, discussions about early detection and rapid response (EDRR), and talks about citizen science and community involvement. My head was swimming with impressions and questions. Clearly there are no easy answers when it comes to invasive species, and like other complex, global issues (made more challenging as more players are involved), the increasingly deep well of issues and concerns to resolve is not likely to ever run dry.

Future posts will dig further into some of the discussions that were had on day three. For now, here are a few resources that I gathered throughout the workshop:

Bering Sea Marine Invasives– With melting ice in the Northwest Passageallowing for more ships to pass through, along with increasing traffic in the Bering Sea resulting simply from the rise of human population, there is increasing concern about the species that will be introduced in the process.

Copper River Delta Science Symposium – “Alaska’s Copper River delta is the largest contiguous wetland on the Pacific Coast of North America and an important region for both fish and wildlife. … [T]he CRDSS will focus on the delta as a system, covering topics from hydrology and geomorphology to avian nesting ecology and trophic relationships.”

Seagrass meadows are found along soft-bottomed, shallow, marine coastlines of every continent except Antartica. Their abundance and the important roles they play earn them the title of third most valuable ecosystem on the planet after estuaries and wetlands. These extensive meadows are made up of a group of flowering plants that are unique in their ability to thrive submerged in salty seawater. Tossed about by the tides, they feed and harbor an incredibly diverse world of marine life and help protect neighboring ecosystems by stabilizing sediments and mitigating pollution.

Seagrasses are often confused with seaweed, but they are very different organisms. Seaweed is algae. Seagrasses are plants that at one point in their evolutionary history lived on land but then retreated back into the waters of their ancient ancestors. They are rooted in the sediment of the sea floor and, depending on the species, can reproduce both sexually (submerged flowers are pollinated with the help of moving water) and/or asexually (via rhizomes). Although many of them have a grass-like appearance, none of them are in the grass family (Poaceae); instead, the approximately 72 different species belong to one of four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae, or Cymodoceaceae).

Seagrass meadows can be composed of a single seagrass species or multiple species, with some meadows consisting of a dozen species or more. Seagrasses depend on light for photosynthesis, so they generally occur in shallow areas. How far seagrass meadows extend out into the ocean depends on light availability and the shade tolerance of the seagrass species. Their presence at the shoreline is limited naturally by how exposed they become at low tide, the frequency and strength of waves and associated turbidity, and low salinity from incoming fresh water.

Seagrass meadows benefit life on earth in many ways. As ecosystem engineers they create habitat and produce food for countless species, sequester a remarkable amount of carbon, and help maintain the health of neighboring estuaries, mangroves, coral reefs, and other ecosystems. They are home to commercial fisheries, which provide food for billions of people. Like many ecosystems on the planet, they are threatened by human activity. Pollution, development, recreation, and climate change jeopardize the health and existence of seagrass meadows. Thus, it is imperative that we learn as much as we can about them so that we are better equipped to protect them.

Turtle grass (Thalassia testudinum) growing in an estuary on the coast of San Salvador Island, Bahamas (photo credit: wikimedia commons)

In a report published in a February 2017 issue of Science, researchers examined the ability of seagrass meadows in Indonesia to remove microbial pathogens deposited into the sea via wastewater. When levels of the bacterial pathogen Enterococcus were compared between seagrass meadows and control sites, a three-fold difference was detected, with the seagrass meadows harboring the lowest levels. When other potential disease-causing bacteria were considered, the researchers found that “the relative abundance of bacterial pathogens in seawater” was 50% lower in both the intertidal flat and the coral reefs found within and adjacent to the seagrass meadows compared to control sites.

This has implications for the health of both humans and coral reefs, the latter of which face many threats including bacterial diseases. Two important coral reef diseases, white syndrome and black band disease, as well as signs of mortality associated with bleaching and sediment deposition “were significantly less on reefs adjacent to seagrass meadows compared to paired reefs,” according to the report.

The researchers note that “seagrasses are valued for nutrient cycling, sediment stabilization, reducing the effects of carbon dioxide elevation, and providing nursery habitat for fisheries.” The results of this study demonstrate the potential for seagrass meadows to “significantly reduce bacterial loads,” benefiting “both humans and other organisms in the environment.” Yet another reason to care about and conserve this vital ecosystem.